Scootrd-
SP stands for "soft point", usually pointed, with an exposed lead tip. SST is a term Hornady uses to describe their pointed bullet with a plastic tip, and boat tail base.
The plastic tip, and boat tail base serve to make the bullet a little "sleeker" when it travels thru the air, a little less wind resistance. This means the bullet will drop an inch or two less at long range. It does NOT mean the bullet is more accurate. Each rifle is a law unto its self when it comes to what shoots good, and many rifles shoot best with round nose, flat base bullets. Thats why I said before find a good controled expansion hunting bullet that shoots well, and be happy.
Now, if you are not a hunter, and enjoy shooting little groups from a bench rest, then concentrate instead on loads that deliver maximum accuracy, and don't be concerned with good game performance. You might be suprised what is the most accurate load in your rifle, you just have to test and experiment. Don't worry about maximum velocity either, usually the most accurate load is a little less then max, and like I said, a buck can't tell the difference, within reason.
The "Light Magnum" load is just a load that Hornady sells that is loaded to about 100 / 150 feet per second faster then normal factory loads with the same weight bullet. Its done by the use of specialy blended powders I believe, so that pressures stay within industry limits. It's really a sales pitch, as the slight increase in velocity does not make a difference in the real world, and accuracy is often no better then any other normal factory load. It does cost a few bucks more...
Larry